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Michel Coornaert

Publications and source records attributed to Michel Coornaert.

At least 19 recordsLinked to original sources
arXiv2026

On surjunctive and injunctive subshifts of finite type

A dynamical system is said to be surjunctive if every injective endomorphism of the system is surjective and it is said to be injunctive if every surjective endomorphism is injective. An endomorphism of a dynamical system is called pre-injective if its restriction to every homoclinicity class of the phase space is injective. One says that a dynamical system has the Moore property if every surjective endomorphism of the system is pre-injective and that it has the Myhill property if every pre-injective endomorphism is surjective. We give characterisations of surjunctivity and injunctivity for $\Z$-subshifts of finite type in terms of their irreducible components and their Cantor-Bendixson decomposition. We also prove that a $\Z$-subshift of finite type is surjunctive if and only if it has the Moore property and that every injunctive $\Z$-subshift of finite type is surjunctive. This implies in particular that a $\Z$-subshift of finite type has the Moore property whenever it has the Myhill property.

math.DS↗
arXiv2025

A Garden of Eden theorem for Smale spaces

Given a dynamical system $(X,f)$ consisting of a compact metrizable space $X$ and a homeomorphism $f \colon X \to X$, an endomorphism of $(X,f)$ is a continuous map of $X$ into itself which commutes with $f$. One says that a dynamical system $(X,f)$ is surjunctive if every injective endomorphism of $(X,f)$ is surjective. An endomorphism of $(X,f)$ is called pre-injective if its restriction to each $f$-homoclinicity class of $X$ is injective. One says that a dynamical system has the Moore property if every surjective endomorphism of the system is pre-injective and that it has the Myhill property if every pre-injective endomorphism is surjective. One says that a dynamical system satisfies the Garden of Eden theorem if it has both the Moore and the Myhill properties. We prove that every irreducible Smale space satisfies the Garden of Eden theorem and that every non-wandering Smale space is surjunctive and has the Moore property.

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arXiv2025

Topological stability of semigroup actions and shadowing

We investigate expansiveness, topological stability, and shadowing for continuous actions of semigroups on compact Hausdorff spaces. We characterize semigroups for which all full shifts are expansive. We show that every expansive continuous monoid action on a compact Hausdorff space which has the shadowing property is topologically stable, and that a subshift with finite alphabet over a monoid has the shadowing property if and only if it is of finite type.

math.DS↗
arXiv2025

Strongly sofic monoids, sofic topological entropy, and surjunctivity

We introduce the class of strongly sofic monoids. This class of monoids strictly contains the class of sofic groups and is a proper subclass of the class of sofic monoids. We define and investigate sofic topological entropy for actions of strongly sofic monoids on compact spaces. We show that sofic topological entropy is a topological conjugacy invariant for such actions and use this fact to prove that every strongly sofic monoid is surjunctive. This means that if $M$ is a strongly sofic monoid and $A$ is a finite alphabet set, then every injective cellular automaton $τ\colon A^M \to A^M$ is surjective. As an application, we prove that the monoid algebra of a strongly sofic monoid with coefficients in an arbitrary field is always stably finite. Our results are extensions to strongly sofic monoids of two previously known properties of sofic groups. The first one is the celebrated Gromov-Weiss theorem asserting that every sofic group is surjunctive. The second is the Elek-Szabó theorem which says that group algebras of sofic groups satisfy Kaplansky's stable finiteness conjecture.

math.GR↗
arXiv2024

Stable finiteness of monoid algebras and surjunctivity

A monoid $M$ is said to be surjunctive if every injective cellular automaton with finite alphabet over $M$ is surjective. We show that monoid algebras of surjunctive monoids are stably finite. In other words, given any field $K$ and any surjunctive monoid $M$, every one-sided invertible square matrix with entries in the monoid algebra $K[M]$ is two-sided invertible. Our proof uses first-order model theory.

math.RA↗
arXiv2023

First-order model theory and Kaplansky's stable finiteness conjecture

Using algebraic geometry methods, the third author proved that the group ring of a surjunctive group with coefficients in a field is always stably finite. In other words, every group satisfying Gottschalk's conjecture also satisfies Kaplansky's stable finiteness conjecture. Here we present an alternative proof of this result based on first-order model theory.

math.GR↗
arXiv2022

Expansive actions with specification of sofic groups, strong topological Markov property, and surjunctivity

A dynamical system is a pair $(X,G)$, where $X$ is a compact metrizable space and $G$ is a countable group acting by homeomorphisms of $X$. An endomorphism of $(X,G)$ is a continuous selfmap of $X$ which commutes with the action of $G$. One says that a dynamical system $(X,G)$ is surjunctive provided that every injective endomorphism of $(X,G)$ is surjective (and therefore is a homeomorphism). We show that when $G$ is sofic, every expansive dynamical system $(X,G)$ with nonnegative sofic topological entropy and satisfying the weak specification and the strong topological Markov properties, is surjunctive.

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arXiv2022

Invariant sets and nilpotency of endomorphisms of algebraic sofic shifts

Let $G$ be a group and let $V$ be an algebraic variety over an algebraically closed field $K$. Let $A$ denote the set of $K$-points of $V$. We introduce algebraic sofic subshifts $Σ\subset A^G$ and study endomorphisms $τ\colon Σ\to Σ$. We generalize several results for dynamical invariant sets and nilpotency of $τ$ that are well known for finite alphabet cellular automata. Under mild assumptions, we prove that $τ$ is nilpotent if and only if its limit set, i.e., the intersection of the images of its iterates, is a singleton. If moreover $G$ is infinite, finitely generated and $Σ$ is topologically mixing, we show that $τ$ is nilpotent if and only if its limit set consists of periodic configurations and has a finite set of alphabet values.

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arXiv2021

On linear shifts of finite type and their endomorphisms

Let $G$ be a group and let $A$ be a finite-dimensional vector space over an arbitrary field $K$. We study finiteness properties of linear subshifts $Σ\subset A^G$ and the dynamical behavior of linear cellular automata $τ\colon Σ\to Σ$. We say that $G$ is of $K$-linear Markov type if, for every finite-dimensional vector space $A$ over $K$, all linear subshifts $Σ\subset A^G$ are of finite type. We show that $G$ is of $K$-linear Markov type if and only if the group algebra $K[G]$ is one-sided Noetherian. We prove that a linear cellular automaton $τ$ is nilpotent if and only if its limit set, i.e., the intersection of the images of its iterates, reduces to the zero configuration. If $G$ is infinite, finitely generated, and $Σ$ is topologically mixing, we show that $τ$ is nilpotent if and only if its limit set is finite-dimensional. A new characterization of the limit set of $τ$ in terms of pre-injectivity is also obtained.

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arXiv2018

On injective endomorphisms of symbolic schemes

Building on the seminal work of Gromov on endomorphisms of symbolic algebraic varieties [10], we introduce a notion of cellular automata over schemes which generalize affine algebraic cellular automata in [7]. We extend known results to this more general setting. We also establish several new ones regarding the closed image property, surjunctivity, reversibility, and invertibility for cellular automata over algebraic varieties with coefficients in an algebraically closed field. As a byproduct, we obtain a negative answer to a question raised in [7] on the existence of a bijective complex affine algebraic cellular automaton $τ\colon A^{\mathbb Z} \to A^{\mathbb Z}$ whose inverse is not algebraic.

math.AG↗
arXiv2018

On the Garden of Eden theorem for endomorphisms of symbolic algebraic varieties

Let $G$ be an amenable group and let $X$ be an irreducible complete algebraic variety over an algebraically closed field $K$. Let $A$ denote the set of $K$-points of $X$ and let $τ\colon A^G \to A^G$ be an algebraic cellular automaton over $(G,X,K)$, that is, a cellular automaton over the group $G$ and the alphabet $A$ whose local defining map is induced by a morphism of $K$-algebraic varieties. We introduce a weak notion of pre-injectivity for algebraic cellular automata, namely $(*)$-pre-injectivity, and prove that $τ$ is surjective if and only if it is $(*)$-pre-injective. In particular, $τ$ has the Myhill property, i.e., is surjective whenever it is pre-injective. Our result gives a positive answer to a question raised by Gromov in~\cite{gromov-esav} and yields an analogue of the classical Moore-Myhill Garden of Eden theorem.

math.DS↗
arXiv2018

Homoclinically expansive actions and a Garden of Eden theorem for harmonic models

Let $Γ$ be a countable Abelian group and $f \in \Z[Γ]$, where $\Z[Γ]$ denotes the integral group ring of $Γ$. Consider the Pontryagin dual $X_f$ of the cyclic $\Z[Γ]$-module $\Z[Γ]/\Z[Γ] f$ and suppose that $f$ is weakly expansive (e.g., $f$ is invertible in $\ell^1(Γ)$, or, when $Γ$ is not virtually $\Z$ or $\Z^2$, $f$ is well-balanced) and that $X_f$ is connected. We prove that if $τ\colon X_f \to X_f$ is a $Γ$-equivariant continuous map, then $τ$ is surjective if and only if the restriction of $τ$ to each $Γ$-homoclinicity class is injective. We also show that this equivalence remains valid in the case when $Γ= \Z^d$ and $f \in \Z[Γ] = \Z[u_1,u_1^{-1}, \ldots, u_d, u_d^{-1}]$ is an irreducible atoral polynomial such that its zero-set $Z(f)$ is contained in the image of the intersection of $[0,1]^d$ and a finite union of hyperplanes in $\R^d$ under the quotient map $\R^d \to \T^d$ (e.g., when $d \geq 2$ such that $Z(f)$ is finite). These two results are analogues of the classical Garden of Eden theorem of Moore and Myhill for cellular automata with finite alphabet over $Γ$.

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arXiv2017

A Garden of Eden theorem for principal algebraic actions

Let $Γ$ be a countable abelian group and $f \in \Z[Γ]$, where $\Z[Γ]$ denotes the integral group ring of $Γ$. Consider the Pontryagin dual $X_f$ of the cyclic $\Z[Γ]$-module $\Z[Γ]/\Z[Γ] f$ and suppose that the natural action of $Γ$ on $X_f$ is expansive and that $X_f$ is connected. We prove that if $τ\colon X_f \to X_f$ is a $Γ$-equivariant continuous map, then $τ$ is surjective if and only if the restriction of $τ$ to each $Γ$-homoclinicity class is injective. This is an analogue of the classical Garden of Eden theorem of Moore and Myhill for cellular automata with finite alphabet over $Γ$.

math.DS↗
arXiv2017

Surjunctivity and topological rigidity of algebraic dynamical systems

Let $X$ be a compact metrizable group and $Γ$ a countable group acting on $X$ by continuous group automorphisms. We give sufficient conditions under which the dynamical system $(X,Γ)$ is surjunctive, i.e., every injective continuous map $τ\colon X \to X$ commuting with the action of $Γ$ is surjective.

math.DS↗
arXiv2016

A note on the surjunctivity of algebraic dynamical systems

Let $X$ be a compact metrizable group and $Γ$ a countable group acting on $X$ by continuous group automorphisms. We give sufficient conditions under which the dynamical system $(X,Γ)$ is surjunctive, i.e., every injective continuous map $τ\colon X \to X$ commuting with the action of $Γ$ is surjective.

math.DS↗
arXiv2016

A Garden of Eden theorem for Anosov diffeomorphisms on tori

Let $f$ be an Anosov diffeomorphism of the $n$-dimensional torus ${\mathbb{T}}^n$ and $τ$ a continuous self-mapping of ${\mathbb{T}}^n$ commuting with $f$. We prove that $τ$ is surjective if and only if the restriction of $τ$ to each homoclinicity class of $f$ is injective.

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